1. Field of the Invention
The present invention relates to a sputter target that eliminates sputtered particles from redepositing on the sidewalls of the target. More particularly, the present invention relates to a sputter target having a sidewall that is itself sputtered so that any redeposited species will be immediately cleaned by ion sputtering.
2. Background of the Related Art
The process of sputtering is a physical vapor deposition (PVD) technique in which a solid metal, such as aluminum, is used as the source (also referred to as the "target"). The metal atoms are produced by dislodging them from the target with high energy ion bombardment. The high energy ions that cause sputtering are those of an inert gas like argon generated by a plasma.
A simplified sectional view of a conventional sputtering chamber is shown in FIG. 1 including a conventional sputtering target. The chamber 10 generally includes a chamber enclosure wall 12, having at least one gas inlet 14. A substrate support pedestal 20 is disposed at the lower end of the chamber 10, and a target 18 is received at the upper end of the chamber 10. The target 18 is electrically isolated from the enclosure wall 12 by an insulative member 22, such as aluminum oxide, which is generally positioned above the lower target surface 24 to avoid the formation of deposits thereon that could form an electrical short circuit between the target 18 and enclosure wall 12. The enclosure wall 12 is preferably grounded, so that a negative voltage may be maintained on the target 18 with respect to the grounded enclosure wall 12. A shield 26 may be suspended within the chamber 10 and include an annular, upturned, wall 28 on which a clamp ring 30 (or perhaps a collimator) may be suspended over the pedestal 20 when the pedestal 20 is retracted downwardly in the chamber 10 as shown in FIG. 1.
In bias sputtering, the chamber is used to form a metal film by sputtering particles from a target 18 onto a substrate 32 being held at a negative potential with respect to the plasma generated by a power source 34. A major portion of the sputtered metals atoms or groups of atoms follow a substantially linear trajectory over a distribution of angles due to the low pressure maintained in the chamber. The gas composition and pressure in the sputtering chamber 10 is typically achieved by evacuating the chamber down to about 10.sup.-7 Torr before back-filling the chamber with argon to a pressure of a few millitorr. At these gas pressures, the pedestal 20 can be raised upward within the chamber so that the distance between the target 18 and the substrate 32 can be less than the mean free path of the argon gas molecules. Therefore, many sputtered particles travel directly to the substrate without a collision.
However, a significant portion of the sputtered particles become scattered in the gas, due to collisions with the gas, electrical field effects and the like. These scattered particles can redeposit onto various surfaces of the chamber, including the walls 12, the insulative member 22, the pedestal 20, the clamp ring 30 or shield member 26, and the target 18 itself. Redeposition of a conductive sputter material onto the insulative member 22 will eventually form an electrical short between the target 18 and the enclosure wall 12 requiring that the chamber by shut down and cleaned. Redeposition onto other chamber surfaces can lead to a buildup of material that will periodically flake off, thereby generating undesired particles that can then cause fatal defects in the integrated circuit.
Efforts aimed at reducing the concentration of particles in sputter chambers have taken many different approaches. For example, one method for managing the particle concentration in a chamber involves the use of sputter shields (such as shield 26 in FIG. 1) that prevent sputtered particles from depositing directly on the chamber walls. The sputter shield is periodically replaced as part of the process kit so that the extent of the buildup is limited. This method can reduce the frequency at which the chamber 10 must be cleaned, but a fraction of the sputtered particles still pass around the shield and eventually form deposits on the chamber walls and components so that cleaning is necessary.
Another method of reducing the concentration of particles in the sputter chamber introduces a cleaning gas that reacts with the deposits. The gaseous by-products of these reactions are removed from the chamber through an exhaust port coupled to a vacuum pump. However, the deposition process must be discontinued during the cleaning process and evacuation of the cleaning and by-product gases from the chamber before the deposition process can resume. Therefore, both of the methods just described are aimed at ridding the chamber of deposits that have already formed, rather than preventing the deposits from ever forming.
The redeposition of sputtered material onto the sidewall of the target itself has also been recognized as a source of undesirable particles in the chamber. Sputtered particles that become scattered in the chamber atmosphere can redeposit onto the side of the target and accumulate to form particles or sheets of the material. Direct current (DC) power is applied to the target during sputter deposition on a substrate and then removed from the target between substrates. The target, as well as the redeposited material, is continually being heated and cooled, thereby subjecting itself to thermal stress. Over a period of time, this stress will cause particles of the material redeposited on the target sidewall to come lose and fall onto the substrate.
One attempt at preventing redeposition on a target sidewall is shown in FIG. 3. The chamber 50 has a target 51 has a partially beveled perimeter edge 52 that becomes part of the target face 54 that is in contact with the plasma and is sputtered. The beveled edge 52 also effectively reduces the surface area of the sidewall 56 on which redeposition can occur. The sidewall is necessary to avoid line-of sight sputtering of particles directly onto the insulator 58 that electrically isolates the target 51 from the enclosure wall 12.
However, despite the foregoing efforts, there remains a need for a sputter chamber that generates fewer particulates that can damage the integrated circuits being formed. More particularly, there is a need for a sputter chamber and sputter target that eliminate redeposition of sputtered particles onto the target sidewall and prevents a buildup of sputtered material on the insulator. It would be desirable if the chamber required no additional equipment and could be installed as replacement process kit.